1. Field of the Invention
This invention relates to apparatus, systems, and methods for monitoring laser light output in thermally assisted magnetic recording disk drives.
2. Description of the Related Art
Hard-disk drives provide data storage for data processing systems in computers and servers, and are becoming increasingly pervasive in media players, digital recorders, and other personal devices. Advances in hard-disk drive technology have made it possible for a user to store an immense amount of digital information on an increasingly small disk, and to selectively retrieve and alter portions of such information almost instantaneously. Particularly, recent developments have simplified hard-disk drive manufacture while yielding increased track densities, thus promoting increased data storage capabilities at reduced costs.
A typical hard-disk drive will include a stack of disks or “platters” mounted on a common spindle. The surfaces of the disks are typically coated with a material that is magnetized and demagnetized in performing read/write functions. A number of read/write heads may be positioned over the disks as the disks are spun to magnetize portions of the disks to write information thereon or detect the magnetized portions to read information there from. A plurality of read/write heads may be used to simultaneously read information from multiple rigid platters that are typically arranged in a vertical stack and rotated as a unit via the spindle.
The read/write heads write information to the disk by creating an electromagnetic field to orient a cluster of magnetic grains, known as a bit, in one direction or the other. In longitudinal magnetic recording media applications, a magnetic recording layer has a magnetic c-axis (or easy axis) parallel to the disk plane. In perpendicular magnetic recording adjustments are being made to adapt the disk media so that the magnetic c-axis of the magnetic recording layer grows perpendicular to the disk plane.
To read information, magnetic patterns detected by the read/write head are converted into a series of pulses which are sent to the logic circuits to be converted to binary data and processed by the rest of the system. To write information on perpendicular recording media, a write element located on the read/write head generates a magnetic write field that travels vertically through the magnetic recording layer and generally returns to the write element through a soft underlayer. In this manner, the write element magnetizes vertical regions, or bits, in the magnetic recording layer.
The read/write heads are typically moved from one track to another by an actuator that is capable of very precise movements. A slider may be interposed between the read/write heads and the actuator in order to provide a degree of flexibility, enabling the read/write heads to “float” on the surface of the disk on a very thin layer of air, or “air bearing,” as the disks spin at a very high speed relative to the read/write heads. The combination of slider and read/write head is often referred to as the head-gimbal assembly (HGA).
To increase the capacity of disk drives, manufacturers are continually striving to reduce the size of bits and the grains that comprise the bits. The ability of individual magnetic grains to be magnetized in one direction or the other, however, poses problems where grains are extremely small. The superparamagnetic effect results when the product of a grain's volume (V) and its anisotropy energy (Ku) falls below a certain value such that the magnetization of that grain may flip spontaneously due to thermal excitations. Where this occurs, data stored on the disk is corrupted. Thus, while it is desirable to make smaller grains to support higher density recording with less noise, grain miniaturization is inherently limited by the superparamagnetic effect.
Magnetic bit thermal stability is dictated by the equation KuV/KBT where Ku is the magnetic anisotropy energy of the magnetic medium, V is the volume of the magnetic grain, KB is Boltzmann's constant, and T is the absolute temperature. To control the superparamagnetic effect, researchers typically attempt to increase the value of the numerator. Where smaller magnetic grain volume V is desired the magnetic anisotropy energy Ku must be increased to avoid the superparamagnetic effect. However, the increase in Ku is limited by the point where coercivity Hc becomes too great for the media to be written by conventional write heads.
One solution to the problems associated with the superparamagnetic effect is thermally-assisted recording (TAR). In TAR, the volume of a magnetic grain can be reduced while still resisting thermal fluctuations at room temperature. As the name suggests, thermally-assisted recording uses a heat source, typically a laser, to increase the temperature of a magnetic bit during writing such that the coercivity of the magnetic media is substantially reduced. By design the coercivity drops to a level which allows the magnetic field from the write head to orient the bit. Once the temperature is reduced to room temperature, the bit is effectively permanently frozen in the written orientation. This enables the use of media that is magnetically stable at room temperature with the very small magnetic grains required for high-density storage.
Over time, as a laser ages the laser light power decreases. In a typical commercial laser, the laser light power is monitored by a photodiode. However, TAR technology does not utilize currently available commercial lasers with photodiodes because of the added bulk associated with the commercial laser and photodiode. Instead, due to the small size constraints, custom lasers without photo diodes are used in TAR technology. Alternatives to current commercial photodiodes such as a custom photodiode are unpromising because of the added costs and complexity associated with adding another element to the read/write head. Additionally, a photodiode added to the read/write head necessitates additional electrical contact pads on the already limited space on the slider.
Electrically conductive traces or leads extend from the read/write head and along the suspension in order to transport electrical signals from the read/write head components to and from drive electronics. The drive electronics interpret signals from the read/write head in order to retrieve data or send the appropriate signals to the read/write head causing it to write information to the disks. In some hard-disk drive suspensions, the traces are integrated with the suspension in order to provide ease of manufacture and high data rate capability. Such suspensions are referred to as integrated lead suspensions (ILS). A typical ILS has at 4-6 six leads routed from the read/write head to the drive electronics. Thermally assisted recording may require 8 leads routed from the read/write head to the drive electronics (2 for the read head, 2 for the write coil, 2 for a thermal fly height control heater, and 2 for powering the laser). This is a large number of electrical leads disposed on a very small area (the front face of a read/write head can be as small as 0.27×0.78 mm).
In view of the foregoing, it is apparent that a need exists for an apparatus, method, and system for measuring laser light power which does not add additional components or costs to the device. To that end, it would be an improvement in the art to provide an apparatus that utilizes existing elements on the magnetic head to measure laser power.